Method of forming a bearing

ABSTRACT

A method of forming a bearing includes adding at least one particulate material to an aqueous dispersion of polytetrafluoroethylene, mixing the constituents to allow the polytetrafluoroethylene to coagulate to form a mush, spreading the mush on to a support, and curing the mush to form a bearing lining layer on the support. The method is characterized in that the partiulate material is in the form of an aqueous colloidal dispersion including small particles and in that the small particles of the particulate material itself cause coagulation of the polytetrafluoroethylene without need for an additional separate coagulant material.

FIELD OF THE INVENTION

This invention is concerned with a method of forming a bearing of thetype which comprises a support on which a bearing lining layer isformed. The bearing then presents a plain surface against which a movingmember, eg a rotating shaft, can move.

BACKGROUND OF THE INVENTION

A conventional method (see for example GB 2 279 998 A) of forming abearing comprises mixing. polytetrafluoroethylene (PTFE) in an aqueousdispersion with a filler, and optionally strengthening fibres and/orother fillers, adding a lubricant to assist in spreading, and adding anaqueous solution of aluminium nitrate or another salt to cause the PTFEto coagulate to form a “mush”. The mush is spread on to a support, eg asintered bronze layer on a steel backing, and is heated to cure the mushto thereby form it into a bearing lining layer on the support. Thebearing lining bearings produced by this method are susceptible tocavitation erosion, ie the tendency for lubrication oil used with thebearing to cause cavities in the lining material. Furthermore, thismethod has the disadvantage that it involves the use of a significantquantity of lubricant which includes a volatile solvent (toluene iscommonly used as the lubricant).

SUMMARY OF THE INVENTION

It is an object of the-present invention to provide a method of forminga bearing which results in a bearing with improved cavitation erosionresistance and wear resistance

The invention provides a method of forming a bearing comprising addingan aqueous colloidal dispersion of at least one particulate material toan aqueous dispersion of polytetrafluoroethylene, the method alsocomprising mixing the dispersions for a period sufficient to allow thepolytetrafluoroethylene to coagulate to form a mush, spreading the mushon to a support, and curing the mush to form a bearing lining layer onthe support.

In a method according to the invention, the aqueous colloidal dispersioncontains small particles which cause the PTFE to coagulate without theaddition of aluminium nitrate or another salt. It is believed that thesmall particles act as seeds around which the PTFE coagulates.Furthermore, it is found that the bearing lining material containingsmall particles has greater cavitation erosion resistance. In addition,it is found that the method can be carried out using no lubricant orsignificantly less lubricant than the conventional method (eg less than1% by volume as opposed to about 10%) and non-volatile lubricants, suchas polyol-based lubricants, can be used.

In a method according to the invention, the particles of the particulatematerial preferably have their median dimension between 1 nanometer and100 nanometers. Most preferably, the median dimension is between 5nanometers and 30 nanometers. The particulate material may be selectedfrom the group consisting of metals, metal oxides, metal sulphides,metal fluorides, metal carbonates, metal phosphates and silica.Possibilities include oxides of aluminium, tin, titanium, iron, zinc,copper and lead, metals such as iron, tin, nickel chromium, copper andzinc, sulphides of cadmium, iron, lead, copper and zinc, cadmiumcarbonate, calcium fluoride, and phosphates of aluminium, cobalt andiron.

In order to increase the strength or other properties, of the bearinglining material, additional fillers may be included in the aqueousdispersion of polytetrafluoroethylene, eg non-colloidal calciumfluoride, fibres of glass or aramid, or lead.

The invention also provides a bearing formed by a method according tothe invention, comprising a support which comprises a sintered bronzesupport, the bearing also comprising a lining layer which is impregnatedinto the support and stands proud thereof, the lining layer comprisingpolytetrafluoroethylene in which particulate material is dispersed,characterised in that the particulate material has a median particledimension of between 1 nanometer and 100 nanometers. The particulatematerial is preferably alumina.

There now follows a detailed description of an illustrative example inaccordance with the invention.

In the illustrative example, one liter of an aqueous dispersion of PTFEwas mixed with 20 ml of a pigment for 5 minutes in a Kenwood mixer. Tothis dispersion, 20 ml of a non-volatile polyol-based lubricant wasadded and the mixture was mixed for a further two minutes. Then, 300 mlof an alumina colloid (20% w/v) was added and mixed for 15 minutes. Thealumina colloid contained alumina particles having a median dimension of20 nanometers. Coagulation of the PTFE occurred during this 15 minuteperiod. The resultant polymer “mush” was then left to stand for 30minutes before being applied to a support which was formed of sinteredbronze mounted on a steel backing. The mush composition was: PTFE 86.1%v/v, alumina 7.2% v/v, and lubricant 6.7% v/v.

The mush was rolled on to the sintered bronze support using a twinrolling mill thereby impregnating the mush into the porosity of thesintered support. The rolling continued until the mush stood proud ofthe support by 25 to 30 microns. The mush was then cured at 360° C. for4.5 minutes.

Samples made by the illustrative example were tested under lubricatedconditions (strut oil) in a cavitation erosion test rig for 60 minutes.The samples were flat and 40 mm×40 mm. The cavitation erosion damage wasinduced using a vibrating amplifying horn operating at an amplitude of15 microns and a 1 mm separation. At the end of the test, it was foundthat the samples did not have the bronze sinter exposed at all, ie thelining layer had not been completely removed anywhere. The test-wasrepeated using samples made by the conventional method referred toabove. In all cases, at least 10% of the lining layer had been removedexposing the sintered support layer and, in some cases, as much as 50%was exposed.

DETAILED DESCRIPTION OF THE INVENTION

A sample of the bearing made by the illustrative example was tested forwear resistance under non-lubricated conditions using a Halley test rig.In this test, a stainless steel roller was rotated against a flat samplefor 24 hours. The volume of material removed from the sample wasrecorded as 0.22 mm³ and also a final width of the wear scar formed bythe roller was measured. The results showed that both the volume ofmaterial removed (0.53 mm³) and the wear scar width were higher forbearings made by the conventional method described above.

The dynamic coefficient of friction measured under dry conditions forthe samples made by the illustrative example was found to be 0.09 asopposed to the 0.1 measured for pure PTFE.

In a variation of the illustrative method, the lubricant was omittedentirely and the quantity of alumina colloid was reduced to 150 ml. Itwas still found to be possible to make satisfactory bearings. In anothervariation still with 150 ml of alumina colloid, the quantity oflubricant was 0.5 to 2 ml (still the polyol-based lubricant). Thebearings were satisfactory and spreadability of the mush was improved.

What is claimed is:
 1. A method of forming a bearing comprising addingat least one particulate material to an aqueous dispersion ofpolytetrafluoroethylene, mixing the constituents to allow thepolytetrafluoroethylene to coagulate to form a mush, spreading the mushon to a support, and curing the mush to form a bearing lining layer onthe support, the method being characterized in that said particulatematerial is in the form of an aqueous colloidal dispersion comprisingsmall particles and in that the small particles of the particulatematerial itself cause coagulation of the polytetrafluoroethylene withoutneed for an additional separate coagulant material.
 2. A methodaccording to claim 1, characterized in that the particles of theparticulate material have their median dimension between 1 nanometer and100 nanometers.
 3. A method according to claim 2, characterized in thatthe median dimension is between 5 nanometers and 30 nanometers.
 4. Amethod according to claim 1, characterized in that additional fillersare included in the aqueous dispersion of polytetrafluoroethylene.
 5. Amethod according to claim 4 wherein the additional fillers are selectedfrom at lease one of the group comprising: non-colloidal calciumfluoride; fibers of glass or aramid; and lead.
 6. A method according toclaim 1 wherein a lubricant is added to the mixture.
 7. A methodaccording to claim 6 wherein the lubricant is less than 1% by volume. 8.A method according to claim 6 wherein the lubricant is non-volatile. 9.A method according to claim 6 wherein the lubricant is a polyol-basedlubricant.